The overall objective of the research supported by this grant is to characterize the effects of metabolic, pharmacologic and mechanical stimuli on the activity of cardiac receptors with nonmyelinated vagal afferents and to determine whether these stimuli can significantly alter the inhibition of the vasomotor center exerted by these cardiac receptors. The discharge of these receptors will be studied in cats with left atrial and left ventricular hypertrophy, resulting from aortic banding. Kittens will be banded in a manner which will cause minimal aortic outflow tract obstruction initially but which will result in progressive supravalvular aortic stenosis as the kittens grow. This will result in left ventricular and left atrial hypertrophy. Based on what is known about the behavior of the myocardium in cardiac chambers so affected, one can make predictions about the effects of hypertrophy on the behavior of receptors located in these chambers. In particular, an increased threshold and reduced sensitivity of these endings may occur. The second goal for the coming year is to continue studies of the reflex effects mediated through cardiopulmonary receptors with vagal and sympathetic afferents. With regard to cardiac receptors with sympathetic afferents, we have recently shown that in dogs with intact spinal cords, these cardiac receptors which are subserved by sympathetic afferents do not elicit excitatory responses previously reported in cats with spinal cord section. Among the studies which we plan for the coming year is to determine whether these endings can mediate excitatory responses in dogs when the spinal cord has been sectioned. We plan to investigate the influence of digitalis on cardiac receptors and on reflex effects mediated through these endings during heart failure.